A great number of foams of polystyrene, polyvinyl chloride, polyethylene, ureaformaldehyde resins, phenol resins, polyurethane, silicones, epoxy resins and other synthetic polymers are known. The properties of these products are essentially determined by the type of polymer, density, pore structure and pore size as well as by the type of incorporated fillers or reinforcing materials. By judicious selection of suitable polymers, foaming agents, fillers, dispersing agents, foam stabilizers, etc. as well as of the conditions for the foaming process, it is possible to obtain closed-pore, open-pore and mixed-pore brittle, tough, or resilient foams for various technical applications (Ullmanns Encyklopaedie der technischen Chemie, Urban and Schwarzenbach, Munich-Berlin, 3d Edition (1964), Vol. 15, p. 184 and following).
Cellulose foams are also known. According to the processes described in German Pat. Nos. 570 894 and 601 435, viscose which may or may not contain different additives, is coagulated by allowing the natural maturing process to take place or by means of acid gases. In these known processes, consideration is given to such additives as fillers, plasticizers, impregnating agents and binders. Fillers are intended in particular to enhance the strength of the foam. Examples of suitable fillers are: textile fiber, excelsior or wood fiber, pulp, animal hair, asbestos, cork pellets, cork powder, saw dust, mechanical wood pulp and inert powders. Plasticizers, for example, glycerin and glycol are added during coagulation to impart to the viscose foam a greater stability and to prevent drying out of the foam. Examples of imprenating agents are those which minimize the flammability of the cellulose, such as ammonium salts and borax and, furthermore, those which inhibit the water vapor sensitivity of the products, for example aluminum soaps, latex, phenol-formaldehyde condensation products, paraffin emulsions and tar products. Binders, which serve to bond water are: gypsum and cement.
The products derived from these known processes are, according to the patents, suitable as building materials, especially as thermal and sound insulation, as packing material and for any type of plastic products.
However, to date only pure cellulosic, open-pore and resilient, soft foams have gained any industrial importance. Fields of application are sponges and sponge wipes for household use. In other areas--e.g. in the building trade--regenerated cellulose foams have so far found no application. There are a number of reasons for this: First of all, there is the high swelling in water, which fundamentally limits application to dry situations. Another problem is that the cellulose, when processed according to the viscose process, undergoes considerable shrinkage. The result is that the foam mass during xanthate cleavage, but especially during drying, is unevenly deformed and compacted, so that until now it has not been possible to manufacture flame-resistant products having a high mineral content and exhibiting both a low density as well as a fine, uniform pore structure. In this connection, it is pointed out that due to the high processing shrinkage of the viscose, it has not been possible heretofore to produce viscose sponges, i.e. blocks of cellulose foam, on a continuous basis.
In known methods for producing viscose sponges, sodium sulfate crystals are introduced into the viscose mass. A pore structure is obtained after coagulation and xanthate separation when the sodium sulfate is removed by dissolving and washing. The strength of the sponges can also be increased by the incorporation of additional cellulose fibers. Viscose sponges contain large cavities as well as regions with smaller pores. However, a structure of this type is unsuitable for some uses such as thermal insulation, because heat transfer is too high due to air convection. Moreover, because of the time required for dissolving and washing, the viscose sponge process does not appear suitable for the production of industrial cellulose foams. Much more favorable are methods such as the well known foam whipping process or modern techniques such as, for example, foaming by evaporation of emulsified blowing agent additives. However, these methods cannot be used where fiber is to be added since an uneven pore structure is produced due to larger bubbles or pores being formed on the fibers than in the areas between fibers.
From the above discussion, it is readily seen that asbestos, a filler used for cellulose foams in the form of long fibers, presents drawbacks when added to the viscose from which regenerated cellulose products are made. Another drawback of asbestos is that the manufacturing of shaped foams, such as fibers or chips, is limited, since viscose mixed with long asbestos fibers cannot be extruded through narrow spinnerets. Furthermore, it would be desirable to produce water resistant foam moldings by mixing chips with a suitable binder, e.g. one based on phenolic resin, whereby the binder causes the entire system to become hydrophobic.
While the use of asbestos of microfibrous structure, such as the commercially available Asbestine, would eliminate the above drawbacks, shrinkage of viscose during the processing, deformation during drying, and an increase in density are objectional properties resulting from the incorporation of Asbestine. Moreover, it is known that breathing asbestos dust will cause lung cancer (J. C. Gilsin, Composites, March 1972, page 57).
Other known mineral fillers for cellulose foams are cement and gypsum (German Pat. No. 601 435; German Pat. No. 570 894). With cement and gypsum the processing shrinkage of viscose is too high (see control example), creating the same drawbacks as Asbestine.
Now, as in the past, cellulose is available as raw material inexpensively and in large quantities. Cellulose offers a suitable basis for the manufacture of flame-resistant industrial products, because it does not melt on burning and thus does not propagate flames as do thermoplastic materials. The objective was therefore to utilize this raw material for serviceable industrial foams.